This invention relates to a process and apparatus for using electrooxidation to treat fluids to remove waste materials such as dissolved metals and organic compounds.
Many organic pollutants entering the Nation's water resources are not effectively removed by biological oxidation in municipal waste water treatment facilities or in natural water courses. Chlorination in some cases results in noxious chlorinated organics being passed on to the consumer. The trend towards increased reuse of water requires more and better ways of removing organic contaminants from water.
In the past, most communities have accepted the odors and colors associated with refractory organics rather than pay for expensive additional treatment using carbon beds. Water quality was measured by gross measures of the organics present.
In the 1970s the public began to become more aware that the levels of contamination were very important. Trace levels of certain organics were related to severe health problems. The Clean Water Act of 1977 expanded the components regulated in waste discharges from the traditional parameters of BOD and suspended solids to the more toxic pollutants. Section 307 of the Act specifically references a list of 65 pollutant classes.
The Clean Water Act of 1977 attempts to improve the general quality of wastewater through more stringent controls on industrial waste discharges. The Act requires the development of pretreatment standards for any pollutant that interfers, passes through, or otherwise is incompatible with publicly owned treatment works. However, standards by themselves do not solve the problem and new and more efficient treatments are needed to selectively treat local pollution problems.
One particularly difficult area of water treatment is how to decontaminate water and wastewater containing small concentrations of toxic substances or pathogenic organisms. Traditional methods are not very selective and tend to be very expensive when applied to the removal of trace levels of refractory contaminants.
Electrochemical reactors are used in a great many different processes including electrochemical synthesis, electrolysis, electrorefining, electrowinning, electrometallurgy, electrogeneration of chemical species, and electrochemical treatment of waste water.
The efficiency of any electrochemical reactor is related closely to the characteristics of the working electrodes. Generally the higher the ratio of active and useful surface area to reactor volume, the better the efficiency of the electrochemical process. This is particularly important in electrochemistry involving such a poor ionic conductor as tap water containing trace levels of noxious organics.
Numerous attempts to improve reactor efficiency involve various arrangements of sheets and the like to create multiplate electrodes separated by small spaces. Apart from the poor reaction area achieved, these structures suffer from a further disadvantage in that the small spaces tend to become blocked and are difficult to arrange for adequate electrolyte flow. Most designs of this type which offer reasonable efficiency include devices for stirring the electrolyte or otherwise forcing the electrolyte through the electrode.
The largest ratios of electrode area to bed volume are achieved by particulate bed electrodes. An example of such a structure is found in U.S. Pat. No. 3,827,964 to Katsuhiro Okubo et al. Unfortunately, poor electrical contacts between the particles and poor electrolyte ionic conductivity create an uneven potential distribution within the operating bed. Thus only part of the bed is at the potential required for efficient operation so that the benefits of large electrode area are largely lost.
The use of beds composed of a plurality of auxiliary electrodes placed between at least one pair of main electrodes is described in U.S. Pat. No. 3,888,756 to Toru Teshima et al. A DC or AC power source is connected to the main electrodes and the resulting ionic current flowing between the main electrodes induces voltages on the surfaces of the auxiliary electrodes. The auxiliary electrodes consist of conductive materials such as small graphite beads which are insulated from the main electrodes by nonconductive materials such as glass or plastic beads. The induced voltages can be used to drive surface electrochemical reactions at high rate.
Examples of the disclosed electrochemical reactions ar the removal of metal ions from an electrolyte solution by deposition onto an electrode or an increased rate of absorption by the graphite auxiliary electrodes of an inorganic such as hypochlorous ions. Because the potential of the numerous auxiliary electrodes lacks a means for control, this approach is very unselective and most of the electrical energy from the power source is expended on useless side reactions and in the generation of heat. Additionally, this type of bed electrode requires the use of relatively large amounts of nonconductive materials in particulate form to prevent any short between the main electrodes. These nonconductive materials serve no electrochemical purpose, occupy volume and tend to impede the flow of the fluid through the bed.